net: usb: aqc111: Fix out-of-bounds accesses in RX fixup

aqc111_rx_fixup() contains several out-of-bounds accesses that can be
triggered by a malicious (or defective) USB device, in particular:

 - The metadata array (desc_offset..desc_offset+2*pkt_count) can be out of bounds,
   causing OOB reads and (on big-endian systems) OOB endianness flips.
 - A packet can overlap the metadata array, causing a later OOB
   endianness flip to corrupt data used by a cloned SKB that has already
   been handed off into the network stack.
 - A packet SKB can be constructed whose tail is far beyond its end,
   causing out-of-bounds heap data to be considered part of the SKB's
   data.

Found doing variant analysis. Tested it with another driver (ax88179_178a), since
I don't have a aqc111 device to test it, but the code looks very similar.

Signed-off-by: Marcin Kozlowski <marcinguy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/aqc111.c

@@ -1102,10 +1102,15 @@ static int aqc111_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 	if (start_of_descs != desc_offset)
 		goto err;
 
-	/* self check desc_offset from header*/
-	if (desc_offset >= skb_len)
+	/* self check desc_offset from header and make sure that the
+	 * bounds of the metadata array are inside the SKB
+	 */
+	if (pkt_count * 2 + desc_offset >= skb_len)
 		goto err;
 
+	/* Packets must not overlap the metadata array */
+	skb_trim(skb, desc_offset);
+
 	if (pkt_count == 0)
 		goto err;